Sizing the e-commerce app marketplaces

Anyone have any info on the real marketing opportunity of listing our firm's app on the Intuit, Shopify, and ShipStation marketplaces? I thought these companies could advise that each month there are "x" companies that look for inventory software and "x" leads are generated as a result, but they are not sharing details.

Do you know of any other means?

-How much traffic to they get?
-What's the CTR for placement on their marketplace homepage?
-How many of their stores have ever downloaded an app from their marketplace?
-What is their growthrate?